1. Field of the Invention
The present invention relates to a method for controlling handover of mobile terminal equipment between base stations in a communication system, such as a cellular system, and in particular to a method for seamless handover of mobile terminal equipment in such a system.
2. Description of the Prior Art
In many communication systems, terminal equipment which can serve a variety of purposes, such as a transmission of speech, video, fax, file, program and/or measurement data, is increasingly being connected wirelessly. A connection to such mobile terminal equipment is typically set up via base stations, which are coupled to the mobile terminal equipment via air interfaces. It is common to in turn couple the base stations to a switching station, via which connections to the mobile terminal equipments are switched. The switching station can be coupled to an external communication network making it possible to establish connections between the mobile terminal equipment and external terminal equipments as well.
The area around a base station in which a wireless connection of a given quality to a mobile terminal equipment is possible via this base station is designated as the radio cell of this base station. In order to supply a larger area with connection opportunities, a number of base stations are usually distributed over the area to be serviced, so that their radio cells form a network which has full coverage, i.e., the network occupies the entire area to be serviced. These radio cells are classified as picocells, microcells, or macrocells, depending on their spatial coverage. The user of mobile terminal equipment which is coupled to the switching station via a first base station can exit the radio cell of this first base station and enter the radio cell of a neighboring base station. In this case, the path of the existing connection must be relocated from the first base station to the neighboring base station. Such a change of the path of the connection is usually referred to as "connection handover". A significant problem is that, during the change of the connection path, a loss-free data transfer within the connection must be guaranteed, and an interruption of the data transmission must be avoided, if possible. An handover without interruption is typically referred to as a "seamless handover", meaning a change which cannot be perceived by the user.
In known methods, a handover is controlled by a central mobile switching assembly, which is provided for this purpose, this assembly functioning as an interface between the base stations and the switching station. The base stations participating in the connection handover must exchange information about the change of the radio cell, together with information for connection management, with the central mobile switching assembly, which then effects a rerouting of the data to be transmitted to the mobile terminal equipment from the base station which is handing over the connection to the base station which is taking over the connection. Arrangements and methods of this type are taught in Cordless Telecommunications in Europe (Tuttlebee, Wally H. W. (Ed); Springer; London, 1990: 43-48,273-275), for example.
Equipment designed for hardwired communication are frequently employed as switching stations. In these cases, the central mobile switching assembly is controlled at the switching station side in a manner like a group of hardwired terminal equipment units. This control must be converted by the connected mobile switching assembly into a suitable control for the management of wireless communication equipment at the side of the connected base stations. In particular, the central mobile switching assembly must independently control or manage all the processes specific to a wireless communication, such as the processes associated with a change of the radio cell. The functionality required for this is generally very comprehensive, and corresponding mobile switching assemblies are thus relatively expensive.